The invention relates to a method of manufacturing a display tube comprising an envelope having a display window, and means of absorbing X-rays, provided on the envelope during a process step.
The invention also relates to a display tube manufactured according to such a method.
A display tube can be used in black-and-white, colour and projection television, apparatus for displaying digits and letters (Data Graphic Display) and in other apparatus.
In conventional display tubes an image is produced by generating an electron beam and deflecting the beam across a luminescing display screen provided on the inside of the display window. When an image is displayed, X-rays are sometimes generated in the display tube. Safety requirements determine the permissible X-ray release, which will hereinafter be termed the limiting value. U.S. Pat. No. 3,562,518 discloses a method of absorbing X-rays released from a display tube by surrounding the neck portion of the envelope with a coating impregnated with bismuth, and surrounding the remaining portion of the envelope almost completely with a coating containing bismuth trioxide. The absorption must be such that the quantity of X-ray release remains below the limiting value.
The display tubes which are developed and manufactured at present increasingly exhibit X-rays produced in operation. Moreover, safety requirements are becoming tighter in general, and are different from country to country. If the tighter safety requirements are to be met, the absorption of X-rays must be increased.
In the case of the display tube described in U.S. Pat. No. 3,562,518, the tighter safety requirements can be met by increasing the quantity of bismuth and bismuth trioxide. However, this leads to an increased cost and to a higher weight of the display tube.